In a written answer to Parliament, Mr Barker said the new calculated costs of the extension of the 43.3p rate for solar PV show the extended scheme will now have a lifetime bill of £750 million and not the £1.5 billion his department first forecast.

he admitted the annual cost to the scheme would now be £50 million, compared to the £25 million if the changes had been allowed to come into effect last December.

The Energy Minister said the big reduction was because the Department of Energy and Climate change had appealed to the Supreme Court in an attempt to overrule an original High Court judgement and the installation figures were up to the contingency March 3 deadline.

But David Hunt of Eco Environments said the “massive margin of error” was another example of “Keystone Kop” policy-making.

“Once again we see that DECC seem to generate forecasts and financial figures based on the message they want to put out,” he added. “Message comes first, then the numbers to match the message.